1. Field of the Invention
The present invention relates to a method for regulating the voltage of a voltage signal. In a general manner, the invention is concerned with applications involving control of a direct-current voltage which is produced from a non-regulated, chopped or rippled direct-current voltage which may be of high value, and which is then rectified. The invention is more particularly applicable to the medical field in which it is necessary to regulate the high voltage of x-ray emitting tubes with a view to achieving enhanced fidelity in the nature of the x-rays produced. The invention nevertheless finds applications in other fields. In the medical field, the invention is more particularly applicable to the field of mammography in which the quality of the high voltage conditions the homogeneity of the x-rays produced.
2. Description of the Prior Art
The principles of high-voltage supply of x-ray emitters are known. In the case of the so-called HF generators, they essentially make use of a direct-current low voltage which is chopped or undulated. This undulated or rippled low voltage is then applied to a step-up transformer and converted by this latter to a rippled high voltage. This rippled high voltage is then rectified and filtered so as to produce the desired direct-current high voltage. A signal which is representative of the high voltage thus produced can be collected by means of a voltage-dividing resistor bridge. This measuring signal is then compared with a reference value and an error signal is determined. Said error signal is then applied to a regulating device so as to maintain the high voltage produced at a reference value. As a general rule, the error signal is applied to a variable-frequency voltage-controlled oscillator (VCO). The variable frequency signal of this oscillator is applied as a control signal to the inverter.
The values usually accepted for the oscillating frequency of the inverter are of the order of 15 KHz. Since these inverters are usually of the thyristor type or consisting of transistors which permit the flow of current in a resonant circuit in one direction and then in the other, two sets of controlled semiconductor components are employed. The pilot circuit which makes use of these sets of semiconductor components delivers a signal having a double frequency of the order of 30 KHz. The duration of a pulse for turning-on the x-ray tube at the moment of radiography, in particular in a mammograph, is typically of the order of 300 ms. Throughout the following description of the invention, this radiographic pulse will be designated as an exposure. During this exposure, the high voltage between the cathode and the anode of the tube must increase to its nominal value as rapidly as possible. Throughout the duration of the exposure, said high voltage must then remain equal to this nominal value as far as possible. During one exposure, the number of pulses delivered by the pilot circuit is, with the values indicated, of the order of ten thousand. The number of pulses of this pilot circuit which is necessary in order to increase the high voltage from zero to the nominal value is usually of the order of 50.
In an inverter, at least one set of semiconductor components is connected to the terminals of a direct-current low-voltage circuit and is connected in series with an oscillating circuit and with a step-up transformer. When the semiconductor components are triggered into conduction by an inverter pulse, ripple is set up in the oscillating circuit and is converted by the step-up transformer. This ripple is then rectified by a rectifier downstream of the step-up transformer and is applied to a circuit for filtering said rectified high voltage. Said filtering circuit is essentially made up of capacitors. Throughout the duration of one exposure, these capacitors are therefore subjected on the one hand to a relatively constant discharge and on the other hand to periodic re-charges related to the arrival of the re-charge ripples. The constant discharge is related to the use of the x-ray tube, namely to the type of image which it is desired to produce.
In other words, the high voltage thus produced undulates and this is the case even if it is regulated. In fact, the undulation or ripple of said high voltage is related to the characteristic frequency of the inverter and that of the control circuit, and in fact disappears only if the x-ray tube has zero output. This is of no interest, however, since it is required to maintain the high voltage of the x-ray tube at a constant value during its output. The above-mentioned ripple of the high voltage produced is a consequence which is inherent in the principle of increase in direct-current voltage with a direct-current voltage supply.
Ripple can be attended by numerous disadvantages in medical radiography equipment. The hardness of x-rays is in fact very strongly affected by the high voltage available at the terminals of the x-ray tube. In fact, from the trough of the ripple of the direct-current high voltage to the crest of said ripple, the dispersion of hardness of the x-rays produced can be such that the available x-ray images are distorted. Their interpretation is unreliable.
In the present invention, the undulatory behavior of the high voltage produced has been studied and it has been possible to determine the fact that the undulatory behavior adopted by professionals up to the present time was not the best. In fact, the tolerated ripple, designated throughout the remainder of this description as .delta.KV, is of the form: EQU .delta.KV=f(KV, mA, E)
In this formula, KV represents the nominal value of the high voltage produced. The value mA represents the output of the tube. Finally, E represents the direct-current low voltage employed for producing the direct-current high voltage. The function f indicated is a complex function which takes into account the discharge KV and mA and the quality of the high-voltage filtering circuit after rectification. The value .delta.KV is normally lower than a certain percentage of KV. This percentage is established by customary practice in the profession. It may be 30% or even more in the case of machines which operate with a tolerated standard of degradation. It can be 4% in mammographs.
If E were perfectly constant, there would be obtained a ripple having an amplitude .delta.KV and having an approximately sawtooth waveform but which is especially constant. In the final analysis, homogeneity of the x-rays produced would be as already known. In actual practice, however, E is not constant. In fact, the direct-current low voltage employed can be a direct-current low voltage obtained from an electrical network in which the voltage is rectified. The direct-current low voltage E thus rectified fluctuates all the more by reason of the fact that the rectified line-supply signal is not even a three-phase line-supply signal but is on the contrary a single-phase line-supply signal. For example, a rectified single-phase 50-cycle line-supply signal produces a low voltage which in turn has 100-Hz ripples (with full-wave rectification). In other words, taking into account the values indicated, said direct-current low voltage oscillates on the order of 30 times during one exposure. The fluctuation in the direct-current low voltage results in faulty operation of the regulating system. In fact, while the characteristic curve of power transfer of an inverter is a function of the frequency (which is used for regulating), said curve is also a function of the voltage admitted at the input of said inverter. In other words, the power transmitted by the inverter at the moment of triggering of a pulse depends on the voltage supplied to said inverter.
The result thereby achieved is that the amplitude of fluctuation of the rippled high voltage is variable at the frequency of fluctuations of the direct-current low-voltage supply. This is related to the conceptual design of the regulating circuits. These circuits involve measurement of the high voltage produced. As soon as the value of the high voltage produced passes below the value of a reference voltage, the inverter is triggered and sends a re-charge pulse. Thus, when the low-voltage supply is low (in the trough of the ripples of said rippled low voltage), the power transmitted by the inverter is of low value. In consequence, the peak of rectified high-voltage ripple is relatively low in this case. On the other hand, if the direct-current low voltage is of high value (at the moment of the crests of the ripples of said rippled low voltage), the power transmitted by the inverter at each pulse is higher and the peak of the ripple of the rippled high voltage is also higher.
In both cases, re-charging of the filtering capacitors of the high-voltage rectifying circuit is initiated as soon as said high voltage becomes equal to the value of the reference voltage. The result thereby achieved is that, although the direct-current high voltage falls back to a value which is always identical at each re-charge pulse, said high voltage attains on the other hand different peak values as a function of the ripples of the low-voltage supply. In other words, the spectrum of the x-rays produced cannot be known in advance. Moreover, .delta.KV is highly dependent on the state of the line-supply voltage and on the resistance of the supply line. This could have had little importance, however, by reason of the relatively large number (30) of periods of the line-supply signal during one exposure. But the foregoing is all the more true since in practice, during exposure, the mean value of the low-voltage supply undergoes a uniform drop, thus impairing the operation of the x-ray tube to a correspondingly greater extent. This type of regulation corresponds to pulsed regulation on minimum and is one of the less satisfactory types.
Another known type of regulation is concerned with linear regulation. In this case also, it can be demonstrated that the peak value of the high voltage produced also fluctuates in the same manner as the ripple of the low-voltage supply although in a proportion of one-half.
It has become apparent in the present invention that, while fluctuations of the low-voltage supply are inevitable, steps should preferably be taken to ensure that the ripple crests or peaks of the high voltage produced are located at a constant value, thus allowing the low-voltage ripple to produce action on the minimum values attained by the high voltage during the successive charges applied by the inverter.
In practice, it is therefore not the reduction in ripple amplitude related to the variation in low-voltage supply which is contemplated in the invention but rather the choice of a constant peak value of the high voltages produced In radiography, it is then certain that there will always be available an equal quantity of hard x-rays in the spectrum, irrespective of the operation of the tube. The output of less hard x-rays then bear the uncontrolled consequences of variations in the high voltage In the final analysis, rather than provide a known dose of less hard x-rays and an unknown dose of hard x-rays, the invention provides for a known dose of hard x-rays and for an unknown dose of less hard x-rays. This automatically results in higher precision of images since hard x-rays are the most conducive to formation of images.
In order to solve this problem which is essentially related to fluctuation of the low-voltage supply, it is necessary to take into account the fluctuations of said low voltage. It would have been feasible to compute in real time, or better still to tabulate, the variations of f(KV,mA,E) as a function of the low-frequency ripples of E. In practice, it is known that, if E varies by 10%, .delta.KV on the other hand can vary by 80%. A solution of this type is therefore possible in theory. But if it is recalled that E undulates at a frequency of 100 Hz, this makes it necessary to retain a shorter computation time than the period of said low-frequency fluctuation, e.g. 1 ms. Taking into account the number of operations to be performed, this computation time is too short to be validly employed (at low cost) in a radiography installation. In consequence, in an improvement of the invention, rather than perform a regulation which takes into account by computation both the variation in output high voltage and the variation in the low-voltage supply, it is preferred to perform the regulation by taking into account only the variations in output high voltage. In this case, however, the amplitude of this fluctuation is measured at each pulse of the inverter in order to apply, in respect of a following pulse of the inverter, a control which takes into account a preceding ripple variation.